This invention relates to a molding machine for encapsulation which comprises a transfer molding press having a plurality of cylinders provided with resin-pressing plungers; and a molding die formed of a plurality of pots corresponding to the cylinders, runners branched off from the pots and cavities connected to the runners through the corresponding gates, and more particularly to a low pressure transfer molding machine adapted for the resin-molding of a large number of semiconductor elements set on a lead frame. Hitherto, semiconductor elements mounted on a lead frame have been resin-molded by a transfer molding press provided with a transfer die having a single pot and a single cylinder provided with resin-pressing plunger (hereinafter referred to as "resin-pressing cylinder"). The conventional resin-molding process comprises throwing preheated thermosetting resin into a pot drilled in an upper die component. The thermosetting resin is pressed by a resin-pressing plunger to be forced out of the pot through runners and gates into the cavities in which the thermosetting resin is fitted. Semiconductor elements mounted on the lead frame and previously received in the cavities are sealed in the resin. The above-mentioned conventional resin-molding die comprises 2 to 10 runners branched off from a single pot and about 20 cavities formed in the respective runners.
In recent years, a low pressure transfer molding machine is increased in capacity, die-working technology is improved, and automation of a resin-molding system is more advanced. As a result, a large capacity molding die is developed which is adapted for the simultaneous resin-molding of a large number of semiconductor elements. Considering, however, the fluidity of resin, a limitation is naturally imposed on a maximum length of a runner. In other words, the maximum runner length should be set at about 30 to 50 cm. It has been shown, therefore, that a single pot formed in a die presents difficulties in effecting the simultaneous resin-molding of a plurality of lead frames on which a large number of semiconductor elements are mounted. For this reason, a novel die has been developed, as set forth in Japanese patent disclosure No. 54-92,059 which is provided with a plurality of pots. A transfer molding press has also been proposed in the patent disclosure No. 54-92,059 which comprises resin-pressing cylinders provided in a number corresponding to the above-mentioned plural pots.
However, it involves considerable difficulties to attempt to mechanically synchronize the timing in which a plurality of plungers of a molding machine comprising the aforesaid molding die and transfer molding press are to be operated, in view of the intrinsic resistance of the respective cylinders and the fluid resistance in the passage of a molding machine. FIG. 1 shows the relationship between the viscosity of a resin thrown into a pot formed in a die and the points of time at which the resin undergoes physical change. Now the resin begins to be hardened at a point of time t1. Then a large number of void spaces appear in the resin filled in the cavities at a point of time t3 for the molding of semiconductor elements than in the resin filled in the cavities at a point of time t2 for the same purpose.
FIG. 2 shows a change with time in the internal pressure of the resin filled in the cavities. Curve .alpha. indicates a normal change with time in the internal pressure of the resin, and curve .beta. denotes changes with time in the internal pressure of the resin which start at a point of time delayed by t0 from that point of time at which the above-mentioned normal change with time begins. Now let it be assumed that two cavities are formed in a transfer molding press. Where two different patterns (represented by curves .alpha., .beta.) of changes with time appear in the internal pressure of resins filled in the two cavities, then two semiconductor elements sealed in the resins are simultaneously taken out of the transfer molding press in a time T after the initial operation of the molding press. After the internal pressure of the resins filled in the two cavities indicates no change with time, namely, reaches an equilibrium state, the resin filled in one of the two cavities is allowed to stand for a length of time t4. The resin filled in the other cavity is allowed to stand for a length of time t5. In this case, a second resin which indicates a history represented by curve .beta. has a lower hot hardness immediately after taken out of one of the two cavities than that of a first resin which indicates a history denoted by curve .alpha.. When handled, therefore, the second resin is undesirably more easily deformed by an external force than the first resin.
Where, for example, two resins indicate different histories namely, changes with time in internal pressure after inserted into the same molding machine, then the resin filled in the die of the molding machine later than the preceding one shows a different hardening behavior from the previous one. Therefore, variations appear in the reliability of semiconductor elements sealed in such two different types of resin. Moreover in recent years, resins are developed which can be hardened more quickly than has been possible in the past. Therefore, variations in a period extending from a point of time at which a resin is poured into the pot of a molding press to a point of time at which the resin is brought into a cavity formed in the molding die prominently affects the reliability of a semiconductor element sealed in the resin. Further, a bonding wire for connecting the electrode of a semiconductor element to an external lead tends to get finer in recent years and consequently is ready to be more easily affected by a history traced by a resin after thrown into the die.